19、(1) V1=gt=40 m/s
a=(V1-V2)/t=(40-2)/19=2 m/s^2
(2) X1=1/2gt^2=80 m
X2=V1t=160 m
X3=(V1^2-V2^2)/2g=79.8 m
X=X1+X2+X3=319.8 m
18、(1) V3=V0+at=3.5 m/s
(2) X=V0t+1/2at^2=12 m
(3) V=V(4.5)=V0+at=2+0.5*4.5=4.25 m/s
15、6.9s 238.05m
16、5 m/s (过程小复杂就不列了)
20、X=V0t+1/2at^2
60=V0*3+1/2a*9 (1)
120=V0*5+1/2a*25 (2)
a=4 m/s
V0=14 m/s^2
V1=V0+at1=26 m/s^2
V2=V0+at2=34 m/s^2